Pumps are devices which impart motion to a fluid, including either a gas, a liquid, or a slurry (mixtures of solids and liquids). One method of imparting motion to a fluid is through the movement of a membrane, piston or diaphragm. An actuator may be used to provide the force necessary to cause the motion or displacement of the membrane, piston or diaphragm. Several prior art pumps utilize a shape memory alloy actuator (See U.S. Pat. No. 5,045,064, "Constant Pressure Implantable Pump Reservoir and U.S. Pat. No. 5,622,484 "Pump Using Shape Memory Alloys".) These actuators, however, typically use a passive element to provide the necessary restoring force for the shape memory alloy actuator.
It is well known that work may be extracted from the class of shape memory alloys (SMAs) which includes the nickel/titanium alloy sold as Nitinol, as well as other metallic alloys such as InTl, CuZn, NiAl, AgZn, AgCd, and others. Numerous shape memory alloys are discussed in H. Funakubo (ed.) Shape Memory Alloys (Gordon and Breach, 1987), which is incorporated herein by reference. Common to these materials is the large change in Young's modulus as each material undergoes a martenistic phase transition from a martenistic phase (at lower temperature) to austenite (at higher temperature). If the material undergoes elongation on heating under compression, or contraction on cooling under tension, mechanical work is provided. It is known that strain may be applied to shape memory alloys, and work may be derived therefrom, in tension, compression, torsion, flexion, etc.
Other types of contractile materials include magnetostrictive material and electroactive contractile materials such as conducting polymers and piezoelectric material. Some common conductive polymers which exhibit conductive electrical properties include, for example, polyaniline, polypyrrole and polyacetylene.